1. Field of the Invention
The present invention relates to a pattern inspection apparatus and a pattern inspection method. For example, it relates to an inspection apparatus and method for inspecting patterns by using a line sensor.
2. Description of Related Art
In recent years, with an increase in high integration and large capacity of large-scale integrated (LSI) circuits, a circuit line width required for semiconductor elements is becoming narrower and narrower. These semiconductor elements are manufactured by exposing and transferring a pattern onto a wafer to form a circuit by means of a reduced projection exposure apparatus, known as a stepper, while using a master or “original” pattern (also called a mask or a reticle, and hereinafter generically referred to as a mask) with a circuit pattern formed thereon. Therefore, in order to manufacture a mask for transfer printing a fine circuit pattern onto a wafer, an electron beam pattern writing apparatus capable of writing or “drawing” fine circuit patterns needs to be employed. The pattern circuits maybe directly written onto a wafer by the pattern writing apparatus. In addition to the writing apparatus using electron beams, a laser beam writing apparatus which uses laser beams to write patterns is also under development.
Since a lot of manufacturing cost is needed for the production of LSI, an improvement in yield is a crucial issue. However, as typified by a DRAM (Dynamic Random Access Memory) of 1 giga-bit class, the order of a pattern constituting the LSI has been changing from submicron to nano-meter. Then, one of major factors that decrease the yield is a pattern defect of a mask used in exposing and transferring an ultrafine pattern onto a semiconductor wafer by a photolithography technique. In recent years, with miniaturization of an LSI pattern formed on a semiconductor wafer, dimensions to be detected as a pattern defect have become extremely small. Thus, a pattern inspection apparatus for inspecting defects of a transfer mask used in manufacturing the LSI needs to be highly accurate.
Incidentally, with development of multimedia technologies, the size of a liquid crystal substrate of an LCD (Liquid Crystal Display) is becoming larger, e.g., 500 mm×600 mm or more, and a pattern of a TFT (Thin Film Transistor) or the like formed on the liquid crystal substrate is becoming finer. Therefore, it is increasingly required to inspect an ultra-fine pattern defect in a large range. For this reason, it is urgently required to develop a pattern inspection apparatus which efficiently inspects defects of a pattern of a large-area LCD and a photomask used in manufacturing the large-area LCD in a short time.
As an inspection method, there is known the method of comparing an optical image obtained by capturing a pattern formed on a target workpiece or “sample” such as a lithography mask at a predetermined magnification by use of a magnification optical system with design data, or comparing it with an optical image of an identical pattern on the target workpiece. For example, the following is known as pattern inspection methods: “die to die inspection” that compares optical image data obtained by capturing images of identical patterns at different positions on the same mask, and “die to database inspection” having the steps of inputting into an inspection apparatus the writing data (design pattern data) generated by converting pattern CAD data into an appropriate format for input to a writing apparatus when writing a pattern on a mask, generating design image data (reference image) based on the input writing data, and comparing the design image data with an optical image serving as measurement data obtained by capturing the image of the pattern. When inspecting using the inspection apparatus, the target workpiece is placed on a stage to be scanned by a flux of light while the stage is moving to perform inspection. The target workpiece is irradiated with a flux of light from a light source and an illumination optical system. Light transmitted through the target workpiece or reflected therefrom is focused on a sensor through the optical system. The image captured by the sensor is transmitted to a comparison circuit as measurement data. In the comparison circuit, after position alignment of the images, the measurement data and the reference data are compared based on an appropriate algorithm. If there is no matching between them, it is judged that a pattern defect exists.
FIGS. 23A and 23B show an example of a method for judging defects. FIG. 23A shows where a defect 84 exists in one of a plurality of light receiving elements 80. In this case, it is supposed the pixel value (gray level value) of the transmitting part is adjusted to be “20” and that of the shading part to be “0”, for example. Then, a pixel 82 with no defect 84 being a shading part has a gray level value “20”, whereas the pixel 82 with imaged defect 84 has a gray level value “10” as shown in FIG. 23B. Although not shown, gray level values of all the pixels in the reference data are respectively “20” because there is no defect 84. If the defect judgment threshold value for judging a defect is defined to be 7, for example, when a difference between the gray level value of a pixel and that of the reference data is 7 or more, it is judged that there is a defect. In the case of FIGS. 23A and 23B, since the pixel 82 having imaged the defect 84 has a gray level value of “10”, it is judged that there is a defect because of being different from the gray level value of the reference data by 7 or more gray levels.
If a defect exists at a position straddling the boundary of a pixel region which is captured by a sensor, inspection sensitivity falls because a part of information of the defect is missing.
FIGS. 24A and 24B show an example of the situation of a defect position straddling the boundary of a pixel region. For example, when the center of a defect 86 straddles the boundary of a pixel region as shown in FIG. 24A, the two straddled light receiving elements 80 respectively capture half of the defect 86. In that case, the pixel 82 without the defect 86 has a gray level value of 20, whereas the pixel 82 having captured half of the defect 86 has a gray level value of 15 as shown in FIG. 24B. As mentioned above, if the defect judgment threshold value is defined to be 7, since the pixel 82 having imaged half of the defect 86 has a gray level value of 15, it differs from the gray level value of the reference data by only 5 gray levels. Thus, it is not judged that there is a defect, thereby making an incorrect judgment.
Now, although not related to the case of a defect straddling the boundary of a pixel region to be captured by a sensor, in the die-to-die inspection method using the inspection apparatus, a technique is disclosed that makes the relation between a pixel of chip A and a pattern be the same as the relation between a pixel of chip A′ to be compared and the pattern (refer to Japanese Patent Application Laid-open (JP-A) No. 2004-212221). Moreover, another technique is disclosed that performs mesh dividing into grids each being smaller than the size of a pixel when generating reference data from design data in the inspection apparatus (refer to Japanese Patent Application Laid-open (JP-A) No. 10-104168).
As mentioned above, if a defect exists at the position straddling the boundary of a pixel region of the sensor, since detect information is distributed into two straddled pixel regions, the inspection sensitivity is decreased, thereby resulting in a problem of overlooking defects.